With increasing concern over the protection of global environment in recent years, energy saving in household electrical appliances has become an urgent issue. One proposed solution to this problem is to use vacuum heat insulating materials in order to eliminate unnecessary heat transfer.
A vacuum heat insulating material is formed of a core member and envelope members which cover the core member. The core member is made of foamed resin, fibrous material, or the like. The vacuum heat insulating material is vacuum-sealed to lower thermal conductivity of gas. The insulation performance can be maintained only by maintaining the vacuum heat insulating material in a vacuum environment. During a long period of operation, however, gases such as air and water vapor may enter into the vacuum heat insulating material through resin layers, which are heat sealed to each other at the edges of the envelope members. This gradually deteriorates the degree of vacuum and hence the insulation performance.
Japanese Patent Unexamined Publication No. 2000-104889 shows a method of producing a vacuum heat insulating material that prevents the deterioration of the vacuum due to the entry of gas or water from outside.
FIG. 9 shows a sectional view of the conventional vacuum heat insulating material. FIG. 10 shows a sectional view of an envelope member of the conventional vacuum heat insulating material. As shown in FIGS. 9 and 10, vacuum heat insulating material 1 is formed of core member 2 and a bag-like envelope consisting of top envelope member 3a and bottom envelope member 3b larger than top envelope member 3a and protruding at an edge. The bag-like envelope is sealed at edge-seal portion 4 and folded portion 5 thereof by using an adhesive layer so as to be maintained in a vacuum. In folded portion 5, one end of bottom wrapper 3b that is protruded from top wrapper 3a is folded back so as to have two stacked sealing layers.
Each of top and bottom envelope members 3a and 3b is formed of top heat seal layer 7 and bottom heat seal layer 8 with aluminum foil layer 6 disposed therebetween. Aluminum foil layer 6 has gas-barrier properties. Top heat seal layer 7 and bottom heat seal layer 8 are made of high-density polyethylene. Bottom heat seal layer 8 and top heat seal layer 7 of top envelope member 3a are sandwiched between two bottom envelope members 3b so as to form folded portion 5. Folded portion 5 is heat sealed to form inner sealing layer 9 and outer sealing layer 10.
The inner sealing layer is prevented from being exposed outside so as to suppress the deterioration of the vacuum of the envelope. As a result, the vacuum heat insulating material can maintain the insulation performance.
Japanese Patent Unexamined Publication No. 2004-197935 shows a method of producing a vacuum heat insulating material as follows: A planar core member is disposed between the respective heat-seal layers of two opposed envelope members having gas-barrier properties. Then, the envelope members, including a portion having the core member disposed therebetween, are pressed under reduced pressure between hot plates made of an elastic body so that the opposed heat-seal layers are heat sealed along the shape of the core member. This method allows the heat-seal layers to have a larger width in the peripheries of the core member. This suppresses the deterioration of the vacuum in the envelope members, thereby maintaining the insulation performance of the vacuum heat insulating material.
It is, however, difficult from a manufacturing standpoint to heat seal folded portion 5 in such a manner as to have two sealing layers: inner sealing layer 9 and outer sealing layer 10 as in the above conventional structure. This may cause wrinkles or sealing defects.
As another problem, top and bottom heat seal layers 7 and 8, which are sealed between inner and outer sealing layers 9 and 10, are required to be made of a material suitable for heat sealing, thereby narrowing the range of materials suitable for surface protection. For example, high-density polyethylene is suitable for heat sealing, but is not for surface protection due to its low strength properties, especially scratch resistance and pierce resistance. As a result, the vacuum heat insulating material may have pinholes when handled inappropriately after its manufacture.
On the other hand, in the method shown in Japanese Patent Unexamined Publication No. 2004-197935, the envelope members, including the portion having the core member disposed therebetween, are heat sealed by being pressed between the hot plates made of an elastic body. The pressure of the hot plates can be effectively applied to the portion of the envelope members that has the core member disposed therebetween, but not to the portions of the envelope members that do not have the core member therebetween. Therefore, the core member needs to be compressible to a thickness of not more than several millimeters. Using a core member having a comparative large thickness may cause the portions of the envelope members that do not have the core member therebetween to be pressed insufficiently, thereby causing defective heat sealing.
If the load of the hot plates is increased in order to apply sufficient pressure to the portions of the envelope members that do not have the core member therebetween, the core member may be compressed too much. As a result, the core member becomes to have a larger solid thermal conductivity, thereby degrading the insulation performance of the vacuum heat insulating material. Moreover, it is difficult to control the pressure applied to the portions of the envelope members that do not have the core member therebetween, that is, the portions where the envelope members are to be heat sealed to each other. This is because the pressure tends to depend on the flexibility and elasticity of the hot plates and the shape and thickness of the core member.